TYPO3 Core

Yes exactly. That is what i actually wanted to request.

Flushing of all caches somtimes takes a few seconds, maybe some kind of maintainance mechanism would be fine.

I would suggest touching a file called maintenance.tmp in typo3conf
when the file is present send 503 header and redirect to a configured static file.
that file is located in the rootdirectory and has a refresh of 15 seconds.

could be dealt with via htaccess or bootstrap process and would make deploying more robust.

When flushing is done unlink maintenance.tmp file and everything is fine again.

What about some feedback on this?

maintenance while flushing caches

when calling ./typo3/cli_dispatch.phpsh core clear-cache-group system true

the second parameter causes the maintenance to take place
this will automatically be called when flushing all caches from installtool

step 1: create a checkfile in typo3temp called maintenance
step 2: sleep for a few seconds, so apache can handle open requests
step 3: flush caches
step 4: rm the maintenance file

in the htaccess there is a check for the maintenance file
if it exists redirect to maintenance.php file in root directory

that file is pretty static, just sending some headers, loading LocalConfiguration for some settings and includes a template, that is configured to a static core file by default

We will be back soon, blabla

additionally a meta-refresh should be present to refresh the page after a few seconds

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Background:
i configured most caches to apc, so when flushing some caches and someone requests the page in the meantime some caches are written to the a file in typo3temp/Cache, for some reason - if that happens - the autoload mechanism is broken and i get many many exceptions thrown for not-found files, typically t4lib_div and so on used in older extensions.
With the behaviour above you could prevent from that.

One more thing i would consider:
When a maintenance file exists or some other flag is set dont execute any tasks

got a weird problem in my system when deploying while some heavy import task is executing, you could prevent that if a maintenance mode would lock tasks

So lets recap:
1) create command to enable maintenance-file, the tool returns only if no tasks are running and the file is created, if some tasks are about to be executed (without error) the script waits until no tasks are active
1) enable maintenance mode (a maintenance.php is called instead of index.php - via htaccess i would recommend) when the file maintenance.enable exists in document root
2) maintenance mode prevents from executing tasks
3) create command to clear caches
4) create command to end maintenance

Some deploy process could then be:
1) connect
2) create maintenance
3) build
4) flush caches
5) end maintenance